In polymer electrolyte fuel cells, proton activity thermodynamically correlates with the relative humidity (RH) as well as water uptake (λ) of the ionomer electrolyte. Proton activity differences between the two electrodes arise from RH control, and the activity terms of the Nernst equation could become significant. We investigated the effect of proton activity on open-circuit potentials (OCPs) and cell performance by controlling the RH of H2/H2 and H2/air cells, and on the kinetic current (ik ) using a rotating disk electrode (RDE). In the H2/H2 cell, OCPs were caused by the RH difference between electrodes; thermodynamic analysis was performed to rationalize the experimental OCP by correlating the RH and λ. The H2/air cell performance was low owing to the high resistance and low exchange current density at RH < 30%. The RDE measurements showed that ik decreased with increasing concentrations (proton activities) of HClO4, H2SO4, and CF3SO3H (TFMSA).

Polymer -electrolyte-fuel-cell open-circuit voltages (OCVs) are exactly defined by equation (1), where cathode and anode proton activities [(aH+)cathode and (aH+)anode, respectively] usually are identical, so the third term in the right-hand side of equation (2) is ignored. OCV=E0+RT/2F*ln(a1/2 O2*(a2 H+)cathode/aH2O)-RT/2F*ln((a2 H+)anode/aH2) (1) =E0+RT/2F*ln(a1/2 O2*aH2/aH2O)+RT/2F*ln((a2 H+)cathode/(a2 H+)anode) (2) Water vapor pressure is a colligative property that fundamentally correlates to electrolyte concentrations in aqueous solutions. Proton activity is a function of acid concentration, such as pH, when electrolytes are acids. In polymer-electrolyte membranes, water vapor pressure and acid concentration are understood as relative humidity (RH) and water uptake (λ), respectively, where λ represents number of water molecules per sulfonic acid molecule. Several investigations have reported the relationship between RH and λ, meaning that proton activities and associated water uptakes are intimately related to RH. In actual fuel cell operation, cathode RH is determined by ambient-atmosphere and/or humidifier RH(s), and anode RH depends on hydrogen-circulator RH. Therefore, RH is not always identical at both electrodes, and the difference between electrode RHs is considerable during dry operation of polymer electrolyte fuel cells. Therefore, the third term in the right-hand side of equation (2) may be significant for dry operation. We measured OCVs when hydrogen was supplied to both electrodes at 80°C. One electrode (A) was fixed at 30% RH, while RH at the other electrode (B) was varied (0, 5, 10, 20, and 30%). Measured OCVs varied from 0 to 75 mV. For fuel cell tests, electrode A was supplied with hydrogen at 30% RH; electrode B, oxygen at 0, 5, 10, 20, and 30% RH. OCVs deviated from that measured when RH at electrode B was 30%, increasing from 0 to 60 mV with decreasing RH at electrode B. Results are also shown in Figure 1. Proton activities of both electrodes were thermodynamically calculated. The Gibbs–Duhem relation was applied to obtain molar Gibbs free energies of water and sulfonic acid, and proton activity coefficient was calculated using the Gibbs free energy of sulfonic acid and the relationship between RH and λ1–4, assuming that protons and sulfonic anions show identical ionic-activity coefficients. OCVs were estimated using the third term in the right-hand side of equation (2). Results are shown in Figure 1. Fuel-cell current–voltage performance was poor when RHs at the anode and cathode were 30 and 20%, respectively. To determine kinetic current, we measured the oxygen-reduction reaction (ORR) using a rotating-disk electrode (RDE) in concentrated-acid aqueous solutions, which modeled catalyst-layer ionomers. Kinetic currents decreased with acid concentrations. References T. A. Zawodzinski, Jr., C. Derouin, S. Radzinski, R. J. Sherman, V. T. Smith, T. E. Springer and S. Gottesfeld , J. Electrochem. Soc., 140,1041 (1993) P. K. Das and A. Z. Weber, Proceedings of the ASME 2013 11th Fuel Cell Science, Engineering and Technology Conference, Fuel Cell 18010 (2013) V. A. Sethuraman, J. W. Weidner, A. T. Haug, S. Motupally,b and L. V. Protsailo, J. Electrochem. Soc., 155, B50 (2008) A. Kusoglu and A. Z. Weber, Chem. Rev., 117, 987 (2017) Figure 1

The proton activity term is typically ignored in the Nernst equation because of the definition of the unit activity of protons within catalyst layers in proton-exchange membrane fuel cells although the relative humidities of an anode (RHA) and a cathode (RHC) can be different. Herein, we investigate the effect of proton activity on the open-circuit voltage (OCV) of a H2/H2 cell by individually controlling RHA and RHC at ≤30%. The OCV was thermodynamically estimated by applying the correlations of the RH and water uptake of a Nafion® membrane. The OCV experimentally increased with an increase in the humidity difference: the highest OCV of 77 mV was observed at RHA 30% and RHC 0%. The electro-osmotic coefficient (ξ) was calculated and found to be 0.73 at 5%–30% RHC and 30% RHA. The kinetic current (i k ) of the oxygen-reduction reaction was measured by the rotating disk electrode method to verify the influence of proton activity (a H+ ). i k was described as i k ∝ a H+ −β , with β values of 0.29 and 0.45 for H2SO4 and CF3SO3H, respectively, at 0.9 V. The results demonstrate that for the dry operation of fuel cells, especially for heavy-duty applications, proton activity effects within ionomers must be considered.